X-ray imaging has long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial, and abdominal images that often include information necessary for a doctor to make an accurate diagnosis. X-ray imaging systems typically include an X-ray source and an X-ray sensor. When having a thoracic X-ray image taken, for example, a patient stands with his or her chest against the X-ray sensor as an X-ray technologist positions the X-ray sensor and the X-ray source at an appropriate height. X-rays produced by the source travel through the patient's chest, and the X-ray sensor then detects the X-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system obtains the detected X-ray energy from the X-ray sensor and prepares a corresponding diagnostic image on a display.
The X-ray sensor may be a conventional screen/film configuration, in which the screen converts the X-rays to light that exposes the film. The X-ray sensor may also be a solid state digital image detector. Digital detectors afford a significantly greater dynamic range than conventional screen/film configurations, typically as much as two to three times greater.
One implementation of a solid state digital X-ray detector may be comprised of an array of semiconductor field-effect transistors (FETs) and photodiodes. Each pair of photodiodes and FETs receive a pixel of photo data. All photodiodes on a column are connected to readout electronics with data lines through the FETs. A FET controller controls the order in which the FETs are turned on and off so that the photodiodes on a row are selected. When the FETs are turned on, a charge to establish the FET “conductive channel” is drawn into the photodiodes from the readout electronics. On top of the photodiodes, there is a layer of scintillation material (scintillator), such as cesium iodide (CsI) that is used to convert X-rays into visible light. The photodiodes are fully charged before an X-ray exposure and under ideal conditions, the parasitic capacitance of the photodiode retains the charge in the absence of light and X-ray exposure. During exposure, the photodiodes discharge. The amount of discharge is proportional to the X-ray dose received. After the X-ray exposure is complete, the diodes are charged again. The amount of charge restored to a diode is equal to that which was discharged by the exposure and is used by an acquisition system to modulate the intensity of the respective pixels in the displayed digital diagnostic image.
The FETs in the X-ray detector act as switches to control the charging of the photodiodes. When a FET is open (off), an associated photodiode is isolated from the readout electronics. When the FET is closed (on), the photodiode is recharged to an initial charge by the readout electronics. Light is emitted by the scintillator in response to received X-rays. The photodiodes sense the emitted light and are partially discharged. Thus, while the FETs are open (off), the photodiodes retain a charge which may be the initial charge, prior to the X-ray exposure, or less charge because the initial charge has been diminished by the light detected by the photodiode during exposure. When a FET is closed (on), a desired voltage across the photodiode is restored. The measured charge amount to re-establish the desired voltage becomes a measure of the X-ray dose integrated by the photodiode during the length of the X-ray exposure.
X-ray images may be used for many purposes. For instance, internal defects in a target object may be detected. Additionally, changes in internal structure or alignment may be determined. Furthermore, the image may show the presence or absence of objects in the target. The information gained from X-ray imaging has applications in many fields, including medicine, industrial inspection, and security.
A FET that operates with ideal results operates as a switch with the state of “open/close” and “off/on.” In practicality, however, the perfect FET does not exist. There will be always some amount of leakage when a FET is in the state of “open/off.” FET leakage occurs because the FET does not turn off completely when Voff is applied and there is still a small amount of current flowing from the diode onto the data line. FET leakage generates a variety of image artifacts.